Add DESTDIR support for the python binding.
Add DESTDIR support for the python binding.

Without this patch, DESTDIR is ignored, so build systems that use a
staging sandbox prior to installing in real-root, such as Gentoo,
cannot build the python binding.

Caveats:

1) I wrote/tested this against the 3.0.2 release, not current HEAD.

2) It might be preferable to do DESTDIR ?= / at the top of the Makefile,
   so that the if/else/fi can be squashed back out.

3) The install_cython target probably needs a similar change; untested.

4) Other bindings might need something similar.

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